1. Technical Field
Described herein is a photodiode having a buried P-N junction. The photodiode may be used in a solid-state photomultiplier that can detect optical or radiation events.
2. Discussion of the Related Art
Photomultiplier tubes (PMTs) are a standard technology for detecting small light pulses. The photomultiplier tube is a vacuum tube technology that uses a photocathode, dynodes, and an anode. PMTs provide excellent performance characteristics in that they have a large gain (˜106) and have a good quantum efficiency over the spectral range determined by the photocathode material. PMTs are limited in that they are bulky, require high voltage, and are susceptible to large magnetic fields and helium, for example.
Silicon avalanche photodiodes (APDs) are an alternative to PMTs in low-light applications that can have a compact size, high quantum efficiency, insensitivity to magnetic fields and the possibility of mass-production using planar processing. Very large area APDs (40 cm2) as well as monolithic APD arrays (28×28 elements, 0.8-mm pixels) have been fabricated at Radiation Monitoring Devices (RMD), of Watertown, Mass., using a planar process. APD arrays are also being manufactured by other vendors such as Perkin-Elmer, of Waltham, Mass., and Hamamatsu, of Hamamatsu, Japan.
The APDs do, however, have some performance limitations. In particular, the gain of most APD designs is not very high. The APDs manufactured by Hamamatsu and Perkin Elmer with reach-through designs exhibit gains in the range of 50-100. The gain of deep-diffused APDs manufactured by RMD is higher (˜1000 or more). These gain values are lower than those for PMTs (with gain ˜106), and additional circuitry may be needed, such as a relatively low-noise preamplifier, to achieve a high signal-to-noise ratio though the use of APDs. Another disadvantage is that the temporal response of most APD designs is also not as fast as that of high-end PMTs. While APDs are promising devices and are playing an important role in low-light applications, alternative silicon based photodetectors (such as the proposed solid-state photomultipliers) are also promising as they provide a gain comparable to PMTs, very high timing resolution and compact size.
The Geiger photodiode (GPD), also termed a single photon counting diode, is based on the avalanche photodiode with a different mode of operation. The response of the GPD is binary for an incident photon with a large gain (˜106) proportional to the diode junction capacitance and the bias above breakdown. The signal response of each GPD is independent of the number of incident photons, and the probability to generate a self-sustained avalanche is dependent on the bias above breakdown. Once the avalanche is induced either thermally or optically, the diode discharges, creating a sizable current. If a ballast resistor is used in series with the GPD, the self-sustained avalanche can be quenched as the voltage drop across the resistor reduces the bias across the diode to the breakdown voltage. The diode will then recharge and wait for the next avalanche event. The table below provides a brief history of the development of the Geiger photodiodes, and the integration of these diodes for fabrication of solid-state photomultipliers.
YearDevelopment1973McIntyre predicts Geiger-mode operation for APDs operatedabove their reverse bias breakdown voltage.1985McIntyre experimentally validates operation of Geiger-modeAPDs. The electric field in the device enables the operation ofAPD devices above their reverse bias breakdown voltage.1993RMD reports on the fabrication of Geiger-mode APDs (GPDs).Reducing the size of the APD decreases the amplitude of thethermal signal, producing lower DCR in GPD devices.1996Cova et al. reports on the single photon avalanchediode response function in relation to fabricateddevices in a silicon process.2001Buzhan et al. report on the fabrication of an SiPM using theMRS process. The integration of resistors in the MRS processenables the fabrication of large arrays with two readout contacts.2004RMD reports on the migration GPD pixels to a commercial CMOS process. The use of commercial CMOS processfacilitates the integration of circuit components, and theuse of an available multi-user service substantiallyreduces the development cost and time.
The solid-state photomultiplier (SSPM) is built from an array of Geiger photodiodes (GPDs). If the GPDs are placed in an array and read out in parallel, the result provides a two-terminal photodetector, where the signal is proportional to the number of GPDs triggered and the incident light intensity. Since the 1970s, solid-state (silicon) photomultipliers have been in development, and by 2001, fabrication of MRS (metal-resistor semiconductor) SSPMs was successful. CMOS (complementary metal-oxide semiconductor) devices were soon to follow. A number of instruments for the medical, scientific, and defense fields are being developed using SSPMs. Broad device characteristic studies have been made discussing the response of SSPMs for detection efficiency and noise terms (cross talk, after pulsing, and dark counts).
When operating the GPD pixel, the detection efficiency (DE) is proportional to the probability of creating a Geiger avalanche, or Geiger probability. The Geiger probability is related to the electric field within a GPD, which is typically very high. The probability is roughly linearly related to the bias above the breakdown voltage until a 100% Geiger probability is reached. The detection efficiency refers to the overall product of the photon absorption, charge collection, and generation of a self-sustained avalanche, i.e., the ratio of detected photons to incident photons.
The three typical noise sources attributed to SSPMs arise from dark counts, cross talk, and after pulsing. Noise from dark counts, or thermally induced Geiger events, is analogous to dark current in conventional solid-state detectors. The contribution of noise from dark counts is expected to scale with the active area of the SSPM detector. The second significant noise term arises from statistical fluctuations in the optical cross talk between pixels in the SSPM. The Geiger discharge from the GPD pixels in the SSPM produces hot carrier emission of photons from the diode, which may cause cross talk between pixels. The third source of noise in the SSPM arises from the statistical fluctuations associated with after pulsing. The avalanche process creates an ensemble of charge across the diode junction, some of which will populate traps in the silicon. As these traps are liberated, the pixel may avalanche again, yielding another pulse (after pulse). Since there is a probability associated with the occurrence of an after pulse, these pulses add to the parallel noise of the device when integrating the signal.
As the proportional response is provided by the number of pixels triggered in the SSPM, there is an upper bound in the SSPM response, since a GPD can trigger once for integration times on the scale of the GPD recharge time. A nonlinear response function developed for SSPMs can be used to understand the effects on the energy resolution as the number of triggered pixels approaches the number of available pixels.